#include<bits/stdc++.h>
#define ll long long
#define ull unsigned long long
using namespace std;

const ll N = 1e6 + 10, INF = 0x3f3f3f3f;

void solve()
{
    ll n;
    while (cin >> n) {
        vector<ll> a;unordered_set<ll> s;
        for (ll i = 1;i <= n;i++) {
            ll x;cin >> x;
            a.push_back(x);
            s.insert(x);
        }
        set<ll> ans;
        for (ll i = 0;i < n;i++)
            for (ll j = i + 1;j < n;j++) {
                ll t = a[i] + a[j];
                if (t % 2 == 0 && s.count(t / 2)) {
                    ans.insert(t / 2);
                }
            }
        cout << ans.size() << '\n';
    }
}

signed main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);cout.tie(0);
#define ONLINE_JUDGE
#ifndef ONLINE_JUDGE
    std::istringstream in(R"()");
    std::cin.rdbuf(in.rdbuf());
#endif
    ll T = 1;
    //cin>>T;
    for (ll i = 1;i <= T;i++) {
        solve();
    }
}